New to Vassal. Tried playing Advanced Civilization and Axis & Allies with a couple of old friends a few states away.

The problem was Vassal seemed to recognize both of them as the same player. In A&A whichever of them signed in last, that would be the side both of them played. Then one would have to resign that side (Ger) so that they could pick up the other side (Japan).

In Adv Civilization, they could both see each other's trade cards (which were hidden to me, as mine were hidden to them).

Best we could come up with was they were both operating from a router on the same IP, as they're room-mates in an apartment.

Is there a work-around for this? Or are we missing something completely?

Sounds like a password issue. Vassal links the side you're on to password, so every player must have a unique password. (Never use a password other players are likely to have, such as a variation of the game name.)